/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/Parameterization/MeshPlanarSymmetry.h
For any vertices in VertexIndices that are on-plane, take the position in VertexPositionsInOut, apply the on-plane constraint, and return the constrained position in VertexPositionsInOut (ie VertexPositionsInOut is updated to new positions)
void ApplySymmetryPlaneConstraints ( const [TArray](API\Runtime\Core\Containers\TArray)< int > & VertexIndices, [TArray](API\Runtime\Core\Containers\TArray)< FVector3d > & VertexPositionsInOut ) const